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Copyright 2002 IEEE. Reprinted from Proceedings of the Eighth International Symposium on High-Performance Computer Architecture, 2002, February 2002, pages 251-262.

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NOTE: At the time of publication, author Milo Martin was affiliated with the University of Wisconsin. Currently (March 2007), he is a faculty member in the Department of Computer and Information Science at the University of Pennsylvania.

Abstract

This paper advocates that cache coherence protocols use a bandwidth adaptive approach to adjust to varied system configurations (e.g., number of processors) and workload behaviors. We propose Bandwidth Adaptive Snooping Hybrid (BASH), a hybrid protocol that ranges from behaving like snooping (by broadcasting requests) when excess bandwidth is available to behaving like a directory protocol (by unicasting requests) when bandwidth is limited. BASH adapts dynamically by probabilistically deciding to broadcast or unicast on a per request basis using a local estimate of recent interconnection network utilization. Simulations of a microbenchmark and commercial and scientific workloads show that BASH robustly performs as well or better than the best of snooping and directory protocols as available bandwidth is varied. By mixing broadcasts and unicasts, BASH outperforms both snooping and directory protocols in the mid-range where a static choice of either is inefficient.